1. Field of the Invention
The invention relates to a step-down gear system.
2. Description of the Related Art
Mechanisms of this kind are known in the technical field under the term "harmonic drive systems" (Dubbel, Taschenbuch fur Maschinenbau, 15th Edition, page 1069; company publication by Harmonic Drive System GmbH, 63225 Langen/Hessen).
These mechanisms essentially comprise three basic units, i.e.:
a) the so-called wave generator, being the actual drive unit. This includes an elliptical core, on which is mounted a ball bearing and which is also provided with the input shaft; PA1 b) the so-called flex spline, which is a fundamentally cylindrical but radially flexible steel bush (rolling bush) having external toothing, in which the elliptical core is arranged for rotation, and finally PA1 c) the so-called circular spline. This part is an internally-toothed stationary support ring, the teeth of which constantly mesh with the elliptically deformable flex spline, i.e. with said deformable steel bush. In this regard, the internally-toothed support ring has a greater number of teeth than the flexible steel bush, two diametrically opposing peripheral sectors of which constantly mesh with the internal toothing of the stationary support ring. As a result of the rotation of the elliptical core in the flexible steel bush, all the teeth of the steel bush are caused to mesh, one after the other, with the teeth of the internal toothing of the stationary support ring during each rotation, as a result of which a rotation of the steel bush by the difference of the number of teeth takes place.
Using these mechanisms which have a very compact structure, it is possible to achieve considerable speed reduction ratios or step-downs. The rotational directions of the drive and of the output are, however, opposite directions. The formula for calculating the respective gear ratios is: ##EQU1##
wherein Z.sub.1 is the number of teeth of the flexible steel bush, and Z.sub.2 the number of teeth of the stationary internally-toothed support ring. According to information received from the manufacturer of such harmonic drive mechanisms, gear ratios of from 1:72 to 1:320 are possible.
Mechanisms of this kind are used primarily in special purpose machines and industrial robots.
In a known design of such an harmonic drive mechanism (EP 0 514 829 A2), the radially flexible steel bush is designed to be pot-shaped and is provided with a stable end wall at an axial spacing from its external toothing. The thin, basically cylindrical, wall of the steel bush is flexibly deformable, such that it is capable of adapting, on the one hand, to the cylindrical shape of the end wall and, on the other hand, flexibly adapting to the rotating elliptical circumference of the core of the mechanism, i.e. the so-called wave generator.
In other designs of such mechanisms, as the transmission element between the flexible steel bush, i.e. the flex spline, and a transmitting wave (DE 39 06 053 C2 and EP 0 309 197 B2), in each case a second toothed ring comprising an internal toothing is provided and this is arranged coaxially relative to the first stationary internally-toothed support ring, and has, at least substantially, the same inside diameter as the stationary support ring, but a number of teeth differing from that of the stationary support ring. In this regard, the teeth of the flexible steel bush mesh both with the stationary support ring and with the rotating toothed ring, namely in a manner such that the rotation of the flexible steel bush is transmitted directly to the rotating toothed ring, moreover at a ratio of 1:1, since the rotating toothed ring has the same number of teeth as the flexible steel bush which, of course, during the rotation of the elliptical drive core, rolls in the gear rim of the stationary support ring.
The steel bush will hereinafter be referred to as a rolling bush.
In principle, the method of functioning of the generic step-down gear system resides in that peripheral surfaces of different lengths roll off on each other slip-free, as a result of which, during rolling, the shorter peripheral surface rotates by the difference in length.
Since all the individual parts of these known mechanisms are of steel or a similar material, and the mechanisms are, as a rule, equipped with radially flexible, i.e. deformable, ball bearings which are arranged between the peripheral surface of the elliptical drive core and the flexible rolling bush, these mechanisms involve considerable manufacturing costs, especially since a high degree of finishing accuracy is a pre-requisite for trouble-free functioning.